In recent years, porous metal oxide materials are expected as materials that can be newly applicable to optical materials, low-dielectric constant materials, thermal insulation materials, medicines (DDS: drug delivery system), molecular probes, catalysts, adsorbent materials, sensors, paints, inks and the like. For example, it is thought that good insulation effect is obtained by adding in the resin film or paint a porous material having air layers therein as a filler, or in particular adding a porous material having pore that is smaller than mean free path of air in atmospheric pressure, namely smaller than 68 nm, for the purpose of improvement of thermal insulation performance.
Patent Document 1 describes a method of producing porous metal oxide particles using micelles of a surfactant as a template, and also describes that porous metal oxide particles having particle size of 20 to 200 nm, containing mesopores having pore size of 1 to 10 nm are obtained.
Patent Document 2 describes a method for synthesizing a mesoporous metal oxide having three-dimensional cubic phase structure using micelles of a surfactant as a template, and also describes that a mesoporous metal oxide having mean pore size of 5 nm is obtained (Example 2 and FIG. 6).
Patent Document 3 describes a method of producing a porous metal oxide material by using water-insoluble polymer particles, and also describes that a porous metal oxide material, in which mesopores form cubic phase and the pore size is substantially uniform within the range of from 5 to 30 nm, is obtained. Further, this also describes an example of producing porous particles having particle size of from 1 to 10 μm by spray-dry process (Example a12 and the like).
In addition, the following applications are considered.
An image display surface of an image display device such as a liquid crystal display, a CRT display, a projection display, a plasma display, an electroluminescence display, a reflection screen and the like is required to have scratch resistance, so as to avoid damage at the time of handling.
In such situation, a general practice is that a hard coating film is installed on a display surface of the image display device to improve the scratch resistance thereof.
The hard coating layer is formed by using a hard coating material, which is cured by an activation energy derived from ultraviolet, electron beam and the like or by heat, and such hard coating material is generally composed of binder components and fine inorganic particles having higher hardness (see Patent Document 4).
In addition, it is required to reduce reflection of light caused by an external light source such as fluorescent lamp and the like for the above described displays and the like, in order to enhance the visibility of the display surface. A possible approach to reduce the reflection of the extraneous light is an adjustment of the refractive index of the hard coating layer. Simply, the refractive index of the hard coating layer is lowered as compared with the refractive index of the display surface of the image display device. Alternatively, an approach for lowering the refractive index of the surface part of the hard coating layer as compared with that of the hard coat layer may be used. Alternatively, there may be another approach, which is providing coat layers from the side of the hard coating layer, the former having higher refractive index and the latter having lower refractive index of which thickness and refractive index are suitably adjusted on the basis of optical calculations in relation to the refractive index of the hard coating layer.
One can think of a method of using a fluorine-based coating material having low refractive index as a method for reducing the refractive index of the hard coating layer, however, such method is not satisfactory in terms of the curability. One can also think of a method of adding inorganic particles having lower refractive index into a binder component as another approach. Examples of the inorganic particles having low refractive index include silica particles and particles of fluoride such as magnesium fluoride, lithium fluoride, aluminum fluoride, calcium fluoride, sodium fluoride and the like; however, since the refractive index of silica is about 1.44, which is not very low, the effect of adding thereof is small. Although the fluoride particles provide low refractive index, it is difficult to stably obtain fine particles having small particle size. Patent Document 5 describes a method of using hollow silica particles as the inorganic particles having low refractive index. By having air layers that have a low refractive index inside the particles, hollow silica particles achieve a low refractive index as particles. Patent Document 6 describes a method of producing hollow silica particles by utilizing calcium carbonate as a template.